The present invention relates generally to power supplies for LED illumination devices. More particularly, the present invention relates to a compact and inexpensive switching power supply with improved power factor correction for efficiently driving one or more LEDs with a stable current.
Referring to FIG. 9, an example of a conventional power supply as known in the art is shown. A commercial AC power source input Vs is rectified by a full-wave rectifier DB and converted into a DC voltage which is smoothed by a smoothing capacitor C1 to supply a constant current to a light-emitting diode (LED) 3 through a buck converter having a switching element Q, an inductor L and a diode D. A capacitor C2 is connected in parallel with the light-emitting diode 3. A current flowing to the light-emitting diode 3 is detected by a current detecting resistor R and a current detecting amplifier 4 and fed back to a control circuit 1. The control circuit 1 generates a PWM signal for turning on/off the switching element Q and controls an ON time of the PWM signal so that a detected value of the current corresponds to a target value.
In this example, and as shown in FIG. 10, because a capacitor input-type rectifying and smoothing circuit is adopted, the input current waveform is not similar to the input voltage waveform but rather exhibits many higher harmonic components. Even if the power consumed by individual power supplies is small, when a plurality of power supplies with a common structure are connected to the same mains power line in parallel, the effect on other equipment may be considerable.
In a second conventional example as shown in FIG. 11, the smoothing capacitor C1 from the example shown in FIG. 9 is omitted. In this case, because input current flows even in a period when an input voltage is low, as shown in a waveform chart of FIG. 3, the input current waveform is substantially similar to the input voltage waveform and exhibits a sinusoidal current waveform.
However, because the pulsating voltage output from the full-wave rectifier DB becomes low in the period when the input voltage from the commercial AC power source Vs is low, current flow to the inductor L is inhibited even when the switching element Q is turned on. For this reason, as represented by a solid line in FIG. 12, a ripple component of a frequency that is twice as large as that of a frequency of the commercial AC input appears in an output current. A broken line represents an output current waveform in the case where the smoothing capacitor C1 exists (FIG. 9) and a solid line represents an output current waveform in the case where the smoothing capacitor C1 is omitted (FIG. 11).
When the smoothing capacitor C1 is omitted as described above, because the ripple component in the current flowing to the light-emitting diode 3 becomes large, for example, a capacitance of the capacitor C2 connected in parallel to the light-emitting diode 3 needs to be increased, which further increases the size of the power supply. Furthermore, certain operations are undesirably delayed such that even after power is turned off, the light-emitting diode 3 remains lit for a period of time.
In a previously known attempt to address this problem, the LED current is made constant and a power factor of an input current is improved by connecting a step-up chopper circuit to a DC output terminal of a full-wave rectifier and feed-forward controlling an ON period of the switching element according to a pulsating voltage, as well as feedback controlling the ON period so as to suppress variation in a detected value of an output current. Accordingly, the step-up chopper output voltage becomes a DC voltage which is higher than a peak input voltage after full-wave rectification, which is suitable to the case where many light-emitting diodes are serially connected and lit, but is however inefficient in the case where only one or a few light-emitting diodes are lit because power loss increases due to a drop in resistance.
A step-down chopper circuit may also be connected to an output stage of the step-up chopper circuit. However, because power conversion is performed in two stages, circuit losses such as for example in the switches increase and the circuit configuration becomes overly complex.
Thus, it is considered that when the step-down chopper circuit is connected to the DC output terminal of the full-wave rectifier DB without passing through the step-up chopper circuit, the circuit configuration becomes simplified and increased circuit losses such as switching losses can be prevented. However, in the step-down chopper circuit, because a difference between a power source voltage and a load voltage is applied to the inductor upon turning on of the switching element, an input current does not flow in a period when the power source voltage is lower than the load voltage, and an ability to improve an input power factor is limited as compared to the case using the step-up chopper circuit.